One major difficulty in the RF portion of OFDM transmitters is the large peak-to-average ratio (PAR) of the signal amplitude or power. A peak in the signal amplitude and power occurs when some or all of the sub-carriers align themselves in phase. In general, though not necessarily, this occurs once every symbol period. The value of the PAR for power is approximately equal to the number of subcarriers:PAR(dB)≈10 log(N),where N is the number of carriers. For example, in the 802.11a/g OFDM standard, a PAR of approximately 17 dB results if the phases of all 48 data carriers line up during a symbol period.
The high PAR complicates the RF portion of an OFDM transmitter when nonlinear transmit power amplifiers (PAs) are used. For signals with a high PAR, a nonlinear PA must have a large backoff in order to operate in its linear range of amplification even at peak signal values. Without sufficient power backoff, the PA introduces nonlinear distortion, which can degrade system performance and impact the spectral properties of the transmitted signal. Distortion is avoided by backing off the operating point of the PA such that the peak signal level is amplified without distortion. However, this backoff reduces the efficiency of the PA, leading to a larger overall power consumption.
PAR reduction techniques inevitably introduce some signal distortion, since they change the transmitted signal to reduce its PAR. Distortion of the signal must meet the error vector magnitude (EVM, the expected standard deviation between signal constellation points before and after PAR reduction) and spectral mask requirements of the system.
The foregoing examples of the related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.